This may be achieveable with scripting - not sure you can manipulate the ammo variable with v118.
Try doing a small test where on start up you set the player's total ammo to 1.
To do this, make a script as follows:
:plrwithinzone=1:setvar=$A 1
Then create a trigger zone and put the new script in the main script slot.
You will need to add a gun to your test level, then when you have picked up your gun, wander into the trigger zone and see if the ammo in the gun changes to 1.
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